Vacuum process for producing a steel for nonageing enameling iron sheets



3,183,078 VACUUM PROCESS FOR PRODUCING A STEEL FOR NONAGEING ENAMELING IRON SHEETS Tadashi Ohtake, Tobata, and Kameo Matsukura, Yawata, Japan, assignors to Yawata Iron & Steel Co., Ltd., Tokyo, Japan, a corporation of Japan No Drawing. Filed Sept. 24, 1962, Ser. No. 225,843

Claims priority, application Japan, Sept. 29, 1961, 36/ 34,893 1 Claim. (CI. 75-49) This invention relates to a process for producing a steel for nonageing enameling iron sheets which are high in hotand cold-workability and enamelability and specifically adapted to one-coat enameling.

It is preferable that iron sheets to be enameled have no such defects as fish-scaling, bubbling, warping and sagging after the enameling and are high in hotand coldworkability and nonageing.

However, with the enameling iron sheets used today, no desirable results have been obtained in cold-drawability. This is to say, such material as a pure iron sheet in which the carbon content is reduced has been used for an iron sheet to be enameled with a view to preventing such defects as fish-scaling, bubbling, warping and sagging after the enameling. However, in this kind of material, with the reduction of the carbon content, the oxygen content will increase so much in inverse proportion to the carbon content that the steel itself will be likely to be stained with nonmetallic inclusions and will not be high in colddrawability. Further, due to nitrogen contained in such iron sheet, a strain ageing effect will be caused, the mechanical properties or specifically the cold-drawability will deteriorate with the lapse of time and strain marks impairing the appearance will be produced in shaping. It is also desirable in the case of an iron sheet to be enameled to fix carbon in the iron sheet by using such alloying element forming a carbide as titanium for the purpose of preventing the above mentioned defects by carbon. However, if titanium corresponding to the carbon and oxygen contents is contained, the iron sheet will become substantially hard and its cold-drawability will be greatly impaired.

Recently, in order to shorten the enameling process, such iron sheet to be enameled as can be finished by onecoat enameling specifically in a white or bright color is desired. But there are so many difiiculties on the problem of adhesion that nothing satisfactory exists. In the conventional iron sheet to be enameled, bubbling, peeling and fish-scaling are specifically likely to occur.

An object of the present invention is to provide a proce'ss for producing a steel for nonageing enameling iron sheets which are high in hotand cold-workability and enamelability.

A further object of the present invention is to provide a steel for enameling iron sheets which are high in adhesion and have no such defects as bubbling, peeling and fish-scaling in one-coat enameling.

Such objects of the present invention can be attained by a process wherein a molten steel refined in an open-hearth furnace, converter or electric furnace is tapped as of less than about 0.04% C. and less than about 0.3% Mn and is vacuum-degassed to be deoxidized and decarburized and thereon titanium in an amount sufficient for the carbon, nitrogen and sulphur present in the steel to form stable titanium compounds is added to the molten steel.

In such case, depending on the circumstances, the molten steel can be deoxidized by adding aluminum after the vacuum-degassing step.

According to the present invention, as substantially all the carbon, nitrogen and sulphur in the steel are present as stable titanium compounds, 21 steel for nonageing enameling iron sheets high in hotand cold-workability and enamelability will be obtained. Further, according to the present invention, a steel for enameling iron sheets which are remarkably improved in adhesion of enamel can be obtained. Therefore, enameling iron sheets adapted to one-coat enameling requiring specifically no ground coating can be made of the steel obtained according to the present invention.

The present invention shall be explained in detail in the following.

Generall-y in producing an iron or steel sheet to be enameled, a steel is made, is teemed, is-then heated in a soaking furnace, is hot-rolled to be of an intermediate thickness, is acid-pickled, is then individually or continuously cold-rolled and is then softened, recrystallized and bright-annealed.

The steel for enameling iron sheets produced according to the present invention has the carbon content reduced to be less than 0.04% at the tapping from an open-hearth furnace, converter or electric furnace and this molten steel is vacuum-degassed before it is teemed. That is to say, according to the present invention, as the carbon content in the molten steel decreases, the oxygen content in the steel will increase in inverse proportion to the carbon contents. Therefore, it is absolutely necessary to carry out the vacuum degassing treatment. For example, according to results of measurement, more than 0.015, 0.20, 0.26, 0.38, 0.70 and 0.100% 0 were contained when the carbon contents were 0.08, 0.07, 0.06, 0.05, 0.04 and 0.03%, respectively. Due to this increasing oxygen content, the aluminum, titanium and others thrown in to improve the non-ageability and enamelability will be consumed so enormously as to be required in large quantitles, and the quality of the steel will be deteriorated by the nonmetallic inclusions. However, the carbon content in the molten steel can be reduced by the steel making operation but the oxygen content can not be thereby reduced. Therefore, the oxygen content must be reduced by the vacuum-degassing treatment. In such case, it will not be always necessary to throw in aluminum. If the degassing is carried out sufiiciently, aluminum need not be thrown in. However, generally, the degassing treatment at a high temperature for a short time can hardly be expected to be perfect. Therefore, in order to prevent titanium from being consumed by the residual oxygen and nitrogen, the preparatory degassing with aluminum is desirable.

Further, the manganese content in the steel produced according to the present invention must be about 0.30% at most. Usually, any steel material is made to contain. more than 0.30% Mn in order to prevent red-hot brittleness due to FeS in hot-rolling. However, if such large amount of manganese is contained, sagging will be likely to occur in enameling. Generally, red-hot brittleness can be avoided by taking such steps as delay and the adjustment of the thickness of the hot-rolled sheet, but they are not basic measures. However, according to the present invention, with the titanium thrown in, the sulphur in the steel will produce stable TiS at the hot-rolling temperature and the hot-brittleness can be avoided wtihout throwing in manganese. Therefore, the manganese in the steel according to the present invention need not be more than 0.3%. Thus the enamelability of the steel according to the present invention is remarkably improved. The most desirable manganese content is less than 0.05%.

Further, in the steel according to the present invention, in order to eliminate solid soluble nitrogen which will cause ageing, it is necessary to contain a nitride producing element. That is to say, in order to form a stable nitride by annealing, about 0.005 to 0.030% acid soluble Al and about 0.05 to 0.20% Ti are contained in the material. According to many results of measurement, the nitrogen 3 content in such material is about 0.0020 to 0.0070% or usually 0.0030%. In order to deposit such nitrogen as a stable nitride for nonageability, it is necessary to throw in the above-mentioned nitride producing agent.

The contents of such unavoidably accompanying elements as silicon, phosphorus and copper in the steel produced according to the present invention had better be low but may be about the same as in the existing coldrolled steel sheet. However, from the viewpoints of hotbrittleness and stabilization with titanium, the sulphur content had better'he lower. In this respect, it is preferably less than about 0.020%. The desirable content of sulphur is less than about 0.010%.

The steel produced according to the present invention may be hot-rolled by any method. The hot-rolling is carried out today mostly with a continuous rolling mill but is not to be limited only to continuous rolling. In view of the fact that the material can be easily made to be of a required final thickness in the next cold-rolling step after the acid pickling, it may be made about 2.0 to 4.5 mm. thick in the hot-rolling.

The cold-rolling can be carried out by many passings such as in a continuously operating tandem strip mill or reversing cold mill. The rate of cold rolling is determined by the optimum grain size for obtaining a proper cold-drawability and the thicknesses of the hot-rolled sheet and produced sheet. It is usually more than 30%. The material may be or may not be passed through an electrolytic cleaning step before it is annealed after being coldrolled.

By annealing, the nitrogen in the steel can be deposited as stable nitrides of AlN and/or TiN.

As evident from the above description, the steel for enameling iron sheets produced according to the present invention contains less than about 0.02% C, less than about 0.30% Mn, about 0.05 to 0.20% Ti and, depending on the circumstances, about 0.005 to 0.030% Al, and such titanium and aluminum contained in this steel are forming stable compounds with the carbon, nitrogen and sulphur in the steel. Though the steel according to the present invention was already of such low carbon content as less than about 0.04% at the tapping it was further decarburized to be of less than about 0.02% C, because, due to the vacuum-treatment, decreased and therefore, in the teeming, the rimming action was likely to occur and the decarburization proceeded. A low carbon steel of less than about 0.02% C is desirable as a steel for enameling iron sheets. In spite of such low carbon content, the content of O is so low that the steel will not be stained with the non-metallic inclusions and the cold-drawability will be improved. Further, as the carbon, nitrogen and sulphur contained in the steel exist as stable titanium or aluminum compounds, they will cause no ageing effect or show no hot-brittleness. The steel is so high in enamelability that no such defects as fish-scaling, bubbling, warping and sagging will be caused at all in enameling. Therefore, the steel according to the present invention is so high in adhesion of enamel that iron sheets to be enameled specifically adapted to one-coat enameling can be made of it.

An example of the present invention is given in the following.

Example:

An open-hearth furnace steel of ladle analysis values of 0.03% C, 0.05% Mn, 0.01% Si, 0.006% P and 0.015% S was vacuum degassed according to the vacuum-degasification method as described in US. Patent 2,929,704. Then aluminum and titanium were thrown into it. The analysis values of the steel thus obtained were 0.010% C, 0.006% Si, 0.04% Mn, 0.002% P, 0.017% S, 0.073% Cu, 0.028% Ni, 0.010% Cr, 0.014% As, 0.001% V, 0.003% Mo, 0.004% W, 0.000% Co, 0.011% Sn, 0.12% Ti, 0.030% 0 0.015% soluble Al and 0.0029% total N. The mechanical properties of the iron sheet made of this steel were as in the following table:

Thickness mm 1.00 Yielding strength kg./mm. 22.8 Tensile strength kg./mm. 32.2 Elongation "percent..- 45.3 Ericsen value mm 10.6 Hardness 42.9

1 IaRockwcll B Scale.

In the above mentioned analysis values, when the content of C was 0.010%, the content of 0 was usually more than 0.1% but, as the vacuum-degassing treatment was carried out, the content of O reduced to be 0.030% or less than about /3, the nonmetallic inclusions which was usually d=about 0.44 to 0.73% reduced to be (1:0.14 to 0.28% or less than V2 (according to the A.S.T.M. point counting method) and the shape became fine. Further, the enameling showed a higher adhesion and enamelability than of any existing pure iron to be enameled in the enamelability test, Du Pont type impact test, bending test, torsion test and sagging test.

What we claim is:

A process of producing an enameling steel having a carbon content less than 0.02% by weight and in which the carbon, nitrogen and sulfur contained in the steel are in the form of titanium compounds, comprising the steps of making steel consisting essentially of less than 0.04% by weight C, less than 0.3% by wt. Mn and the remainder Fe in a metal refining furnace, tapping molten steel from said metal refining furnace, vacuum degasifying said molten steel to produce a steel to produce a steel containing less than 0.02% by wt. C, less than 0.05% by wt. Mn, 0.020% by wt. S and ODOZO to 0.0070% by wt. N and the remainder Fe, and adding 0.005 to 0.030% by wt. Al and 0.05 to 0.20% by wt. T1 to the thus vacuum-degasificd steel to produce nitrides of the added metals.

References Cited by the Examiner UNITED STATES PATENTS 9/18 Yensen -49 12/55 Morgan 7549 

